Over the past years there has been a revolution in the electronics industry in which the physical size of circuits has been dramatically reduced. Using current technology it is possible to form on a substrate or chip only a few millimeters square 1000 or more circuits. Each circuit formed on the chip may contain numerous transistors, capacitors, resistors and the like. The miniaturization of the circuits has resulted in a sharp decrease in the cost of manufacturing the circuits, and a substantial increase in the speed at which logic functions can be performed by the circuits.
The rapid progress which has been made in the miniaturization of electronic circuits has been the result of the development of fabrication procedures which allow extremely accurate reproduction of very small circuits on the surface of the substrate.
A process which has heretofore been utilized to reproduce the desired circuits on the chips is photolithography. In photolithography a polymeric film which is sensitive to light, generally termed a photoresist, is applied to a support substrate. The film is then exposed to light through a mask which defines a pattern desired to be reproduced on the support. The term light, as used in this specification, refers to light in the visible range and ultraviolet radiation. Exposure of the photoresist results in a difference in solubility between the exposed and unexposed portions. A solvent is used to remove the more soluble portion of the photoresist and a surface relief pattern is obtained. Then, by using suitable etchants, or the like, desired features can be produced on or in the substrate.
It has been found that an electron beam can be used to delineate circuits with greater resolution than that which can be obtained with light. The higher resolution of the electron beam is due, in part, to the fact that an electron beam can be more precisely focused than can a light beam. The electron beam has been proven commercially, and continues to be a viable and important tool for the manufacture of microelectronic circuits and masks for forming printed circuits by other methods.
A more recent approach, which is still under active development, is the use of x-ray radiation to obtain the fine-line resolution required in microelectronic circuits.
One of the factors which, to date, has inhibited the full utilization of electron beam and x-ray lithography in the manufacture of microelectronic circuits is the limitations of the resists which have heretofore been available.
Materials which are suitable for use as resists in either electron beam or x-ray lithography must be carefully selected. The first requirement is that the material must be soluble in a solvent so that it can be applied as a thin film on the supporting substrate. It likewise must have sufficient adhesion to form a tight bond with the substrate. The most important property, however, is that as a result of exposure to the selected type radiation there is a substantial change in the solubility of the exposed and unexposed portions. This is important in that when a solvent, commonly called a developer, is applied to the exposed film of resist, it must selectively dissolve and remove substantially all of the more soluble portion to provide the desired surface relief pattern.
Various polymeric compositions have heretofore been suggested as candidates for resists. In U.S. Pat. No. 3,893,127 there is a disclosure of copolymers of an olefin and sulfur dioxide for use as an electron beam resist. In U.S. Pat. No. 4,061,829 there is a disclosure of chlorine- or bromine-substituted olefins and, in particular, acrylates for use as electron beam and x-ray resists.